The present invention generally relates to a copying machine having a capability of reproducing an image at different magnifications one at a time and, more particularly, to an optical member drive system for the copying machine for repositioning the optical member according to the choice of one of the magnifications.
The term "magnification" hereinabove and hereinafter used is to be understood as interchangeable with a term "magnification ratio" and connotating, along with its literal meaning of "the state of being reproduced on an enlarged scale", both the state of being reproduced on a reduced scale and the equal size reproduction depending on a particular value taken by the ratio.
In the prior art copying machines of a type having a capability of reproducing an image at different magnifications one at a time, a drive motor for driving the objective lens assembly from one position to another according to the choice of one of the magnifications is liable to receive a varying load depending on the position of the lens assembly. More specifically, in the prior art copying machine employing a so-called corner-to-corner alignment method of document placement wherein any original or document desired to be copied is placed on a transparent original support with one corner thereof having been aligned with a reference corner of the original support regardless of the size of the original and the choice of magnifications, the lens assembly is driven by a stepper motor so as to move not only in a direction parallel to the optical axis of the lens assembly, but also in a direction laterally of the optical axis, when the magnification is changed, thereby to adjust the length of the optical path and the position of the optical axis which are appropriate to the newly chosen magnification. The movement of the lens assembly from one position to a new position necessitated by the change in magnification is guided by a magnification varying cam member having a curved guide so shaped and so curved that, while the lens assembly is laterally urged towards the cam member, the lens assembly can be driven by the stepper motor to move along a path conforming to the curved guide. Accordingly, the force loaded on the lens assembly varies depending on whether the direction of movement of the lens assembly lies in the direction in which the lens assembly approaches the curved guide or whether the direction of movement of the lens assembly lies in the direction in which the lens assembly leaves away from the curved guide. Specifically, the force acting on the lens assembly is smaller when the direction of movement of the lens assembly lies in the direction in which the lens assembly approaches the curved guide than when the same lies in the direction in which the lens assembly leaves away from the curved guide. In addition, the force acting on the lens assembly varies, too, depending on the position of the lens assembly. Therefore, the stepper motor is liable to receive a varying load.
Thus, in the prior art optical drive system hereinabove discussed, there is a problem in that, when the load imposed on the stepper motor is excessive, disorder may take place, and conversely, when the load on the lens assembly is too small, quivering may take place.
It is to be noted that the change in load on the stepper motor occurs not only in the copying machine employing the corner-to-corner alignment method of document placement discussed hereinabove, but also in the copying machine wherein, for example, the mirror adapted to be moved for changing the conjugate length is driven together with the lens assembly by a single and common motor.